How Much Concrete Do I Need? The Estimation Guide for Slabs, Footings, and Posts

Ordering concrete short is the most expensive mistake in DIY flatwork — a second truck carries a $150 to $250 short-load fee, and a cold joint where the first pour set up waiting for it. Answering “how much concrete do I need” correctly comes down to one formula applied carefully: length times width times depth, converted to cubic yards, plus a waste factor. This guide walks the math for the three project shapes that cover 95 percent of residential work — slabs, footings, and post holes — then shows where calculators quietly go wrong and when to switch from bags to a ready-mix truck.

The Core Formula, in Feet and Yards

Concrete is sold by the cubic yard (27 cubic feet). The universal calculation:

1. Measure length and width in feet; convert depth to feet (4 inches = 0.33 ft, 6 inches = 0.5 ft).
2. Multiply: length x width x depth = cubic feet.
3. Divide by 27 for cubic yards.
4. Add 10 percent waste (more on this below).

Example: a 12 x 20-foot patio at 4 inches thick is 12 x 20 x 0.33 = 79.2 cubic feet, or 2.93 yards; with 10 percent waste, order 3.25 yards — which a supplier rounds to 3.5. That rounding to the nearest quarter yard is standard, and rounding up is always the right direction.

Slabs: The Depth Decision Drives Everything

Slab volume errors almost always hide in depth, not length and width. Standard residential thicknesses: 4 inches for patios, walkways, and shed slabs; 5 to 6 inches for driveways (6 where trucks park); 6-plus inches for garage slabs and equipment pads. A slab formed at “4 inches” over ungraded, wavy subgrade routinely averages 4.5 to 5 inches — a 15 percent hidden overage on its own. Measure depth at a dozen points off a stringline across your forms and use the average, and if the site slopes, remember thickened edges and any haunches or integral footings must be calculated as separate rectangles and added in. A 24 x 24 garage slab at a true 6 inches is 288 cubic feet = 10.7 yards; order 12.

Footings and Curbs: Long Skinny Rectangles

Footings are the same math with the dimensions rearranged: width x depth x total linear feet. A typical deck or wall footing at 16 inches wide by 8 inches deep is 1.33 x 0.67 = 0.89 square feet of cross-section; multiply by 40 linear feet and you get 35.6 cubic feet, or 1.32 yards — order 1.5. Trenched footings deserve a heavier waste factor (15 to 20 percent) because earth walls are never as clean as forms; soft spots and over-dig swallow concrete invisibly. Frost depth rules the vertical number: in the North, footing bottoms sit 36 to 48 inches down per local code, and every extra inch of depth in the trench is volume you must count.

Post Holes: Cylinders and the Sonotube Shortcut

Round holes use pi: radius squared x 3.14 x depth, in feet. A 12-inch-diameter hole (0.5-foot radius) at 3 feet deep holds 0.5 x 0.5 x 3.14 x 3 = 2.36 cubic feet — subtract the post’s own displacement (a 4×4 takes up about 0.4 cubic feet of that) and you are near 2 cubic feet per hole, which is conveniently one 80-pound bag short of three: an 80-pound bag yields 0.60 cubic feet, so figure three-plus bags per 12 x 36-inch hole. Common quick counts: 8-inch x 24-inch hole, about 1 bag; 10 x 30, about 2; 12 x 48 for tall fences, 4 to 5. Sonotube footings for decks calculate the same way — a 12-inch tube runs 0.79 cubic feet per foot of height.

Using an Online Calculator Without Getting Burned

Concrete calculators (every ready-mix supplier hosts one) do the arithmetic fine; their failure mode is the inputs. Walk through one properly: enter average measured depth, not nominal; split L-shaped or multi-thickness pours into separate simple rectangles and sum them; and check whether the tool adds waste automatically — most do not. Then apply the waste factor yourself: 5 to 10 percent for clean formed slabs on compacted, plate-tamped base; 10 percent standard; 15 to 20 percent for trenched footings, pours over rough grade, or pump jobs (the pump line itself holds concrete). Never let the calculator’s two-decimal answer create false confidence — the supplier’s quarter-yard rounding and your subgrade are both blunter instruments than the math.

Bags or a Truck? The Crossover Point

Bagged concrete makes sense up to roughly one cubic yard — about 45 80-pound bags — beyond which mixing labor and cost both tip decisively toward ready-mix. The full bag-count arithmetic, yields per bag size, and cost comparison live in our how many bags of concrete in a yard guide, and current per-yard truck pricing, short-load fees, and delivery minimums are broken down in our concrete cost per yard guide. The short version: a 10 x 10 x 4-inch pad (1.23 yards) sits right at the crossover; anything bigger, call the plant. When ordering, give the dispatcher your dimensions and let them check your math — they estimate a hundred jobs a week and will catch a depth error on the phone for free.

Steps, Stairs, and Odd Shapes

Steps intimidate estimators more than they should — they are just stacked rectangles. Calculate each step as its own slab (tread depth x width x riser height) and sum them, or use the shortcut for a solid stair block: treat it as a triangle-topped rectangle, half the enclosing box’s volume plus the bottom slab. A typical three-step entry stair, 4 feet wide with 12-inch treads and 7-inch risers, works out to roughly 0.5 cubic yards — call it 0.75 with waste, or about 25 80-pound bags, which is exactly the kind of job where bags still make sense. Circular pads use the cylinder formula from the post-hole section; L- and T-shaped patios split into two rectangles at the inside corner. The only genuinely tricky shape is a free-form curve, which you estimate by averaging several width measurements along the length — measure every 2 feet, average, and treat it as one rectangle at the average width.

Ordering From the Plant: The Questions They’ll Ask

A ready-mix order is more than a yardage number, and knowing the vocabulary gets you contractor treatment. The dispatcher will want: total yards (rounded to the quarter), the mix strength in psi (3,000 for footings and interior slabs, 3,500 to 4,000 for driveways and anything freeze-thaw exposed), slump (4 to 5 inches is workable for hand-finished flatwork), whether you need air entrainment (yes, in any climate with freezing — it is cheap insurance against surface scaling), fiber or other admixtures, and the truck’s access — trucks weigh 60,000-plus pounds loaded, will crack a residential driveway they park on, and their chutes reach only 12 to 16 feet, beyond which you are wheelbarrowing or paying $180 to $250 per hour for a pump. Mention your unload time honestly: most plants include 5 to 7 minutes per yard, with standby fees after. Order for arrival early morning in summer so the pour beats the heat, and never schedule concrete when temperatures will drop below 40°F within 48 hours unless you have a cold-weather plan.

Field Rules That Save the Pour

Three habits separate smooth pours from panicked ones. First, always round up to the next quarter yard and think of the overage as insurance, not waste — have a wheelbarrow-sized side project formed up (a stepping-stone mold, a post base) to receive the extra. Second, verify subgrade before ordering: compacted, level base at the right elevation is the only thing that makes your depth number true. Third, for pours over 3 yards, tell the supplier your pour rate and crew size so trucks are spaced right; concrete waits for no one at 90 degrees in July. Measure honestly, add ten percent, round up — that is the entire craft of the estimate.